A mobile telecommunications system, such as a cellular telephone system, provides air interface channels over which a user can communicate. In the case of a conventional cellular telephone system, a user communicates over a radio channel with a base station, which then connects the user to another party. The type of channels which are used depends on the telephone system. For example, the GSM system uses a hybrid TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access) scheme, in which each channel represents a particular time slot, that is, a regularly recurring time interval, on a specific one of the many frequencies allocated for traffic.
A feature common to all such communications systems is that the total number of available channels is far smaller than the total number of subscribers who could theoretically make calls. Network operators must decide how many channels to make available in each geographical area, given that there is a cost associated with providing such channels.
This means that, when a user wishes to communicate over a channel, there is a possibility that there will be no channel available for such use. Clearly, this is inconvenient for the user.
It is advantageous for the network operator to provide the best possible service to certain favoured users or categories of users, for example those users who make frequent use of the service, thus providing high revenues for the network operator, or those users who are prepared to pay higher charges in return for a higher guaranteed grade of service. Thus, it is advantageous for the network operator to have a tool which minimises the possibility that such a user will find no channel available when he wishes to use the system.
For that purpose, it is known to reserve in each cell a group of channels, specifically for use by favoured users. This is described in the existing IS-136 standard.
However, this has the limitation that the reserved channels are unavailable for use by other users, even if no favoured users are active in the area, while the number of reserved channels may be inadequate if there is a high number of active favoured users.